a. Field of the Invention
The invention relates to the field of acoustic transducers generally, and specifically to a gas adsorber material for use in acoustic transducers.
b. Background Art
The use of porous materials as gas adsorber in loudspeakers to reduce the resonant frequency and/or to virtually enlarge the back volume (i.e., the space behind the loudspeaker diaphragm) is known in the prior art. Adsorbency is a property of a material that causes molecules, either solid or liquid, to accumulate on the surface of the material. The number of molecules adsorbed depends on both the concentration of molecules surrounding the adsorbent material and the surface area of the adsorbent material. An increase in the concentration of molecules surrounding the adsorbent material results in an increase in the number of molecules adsorbed. Similarly, an increase in the surface area also results in a larger number of molecules being adsorbed. An increase in the adsorbency of a gas adsorber located in a loudspeaker back volume will result in a greater reduction of the resonant frequency and/or a greater virtual enlargement of the back volume, providing greater acoustic performance to the loudspeaker.
The technique of virtually enlarging the back volume of a loudspeaker by using a gas adsorber is particularly useful in mobile devices such as mobile telephones, tablets and laptops where the space available as a loudspeaker back volume can be extremely limited. As more features and capabilities are added to mobile devices, the available space for use as a loudspeaker back volume is more scarce. The known methods of the prior art do not provide sufficient adsorbency for the decreased back volume sizes in some newer mobile devices. Further, there is a desire to provide mobile devices having loudspeakers with improved acoustic performance. An increased adsorbency of the gas adsorber material used in the back volume will allow the size of the back volume to be reduced without a reduction in acoustic performance. Alternatively, for a fixed back volume size, an increase in the adsorbency can improve a loudspeaker's acoustic performance.
Various porous materials and different configurations have been used as a gas adsorber material in a loudspeaker back volume to improve the acoustic performance of the loudspeaker. For example, U.S. Pat. No. 4,657,108 teaches the use of activated charcoal granules in a loudspeaker. U.S. Pat. Publ. No. 2011/0048844 A1, the entire disclosure of which is hereby incorporated by reference, also discloses the use of activated charcoal as well as other highly porous materials including Silica, SiO2, Al2O3, Zirconia ZrO3, Magnesia (MgO), carbon nanotubes and fullerene. Still further, U.S. Pat. Publ. No. 2013/0170687 A1 discloses the use of a zeolite material having a silicon to aluminum mass ratio of at least 200.
Loose particles of various porous materials, in powder or fiber form, have been used as gas adsorber materials in loudspeaker back volumes to improve acoustic performance. However, using powders and fibers gives rise to a number of problems. For example, electrically conductive materials, such as activated carbon, can cause shorts if the particles get into the surrounding electrical circuits. Loose powder or fiber can also be displaced by sound waves, reducing the overall adsorption effect of the material. Loose debris can also clog acoustic units and block air paths. Furthermore, certain noble porous material can cause corrosion of metal parts that it may come in contact with, such as the metal housing of a device.
Various methods and structures to overcome the problems of using loose particles of porous material have been developed. For example, U.S. Pat. Publ. No. 2011/0048844 A1 discloses the use of a woven or non-woven fabric made of hydrophobic material to support particles of a porous material such as activated carbon. The fabric container is flexible and can be made to fit in a variety of different spaces. However, such a fabric container does not always provide the optimal amount of gas adsorbing material that can fit within a given volume in a loudspeaker.
U.S. Pat. Publ. No. 2013/0341118 A1 discloses a container for holding a porous material, where the container has at least one wall made of a sound transparent material, such as a filter. The container can have a predetermined three-dimensional shape, such as to conform to the available space within the back volume of a loudspeaker enclosure inside a mobile device, with one wall being made from the sound transparent material to allow for the transfer of sound to the gas adsorption material inside the container.
Whether used in a container or not, one issue faced with the using loose particles of a gas adsorbing material is that the particles can become compacted against each other, impeding any airflow between the particles. This can inhibit air from reaching the surfaces of the particles on the inside of a mass of particles, decreasing the amount of overall surface area exposed to the air inside the back volume.
An issue with employing a container for the gas adsorber is that the packaging itself must utilize some of the available space inside the loudspeaker back volume. Since adsorbency is increased with more surface area exposed to the air, it is desirable to place as much of the gas adsorbing material in the back volume as possible. Thus, attempts have been made to provide a gas adsorbing material in the back volume without the need for a container, while also addressing the problems associated with loose particles.
In the context of large conventional speaker systems, European Pat. Publ. No. EP2003924 A1 attempts to address the problems of compacted loose particles. Disclosed therein is a molded gas adsorber obtained by adding a binder to a plurality of particles of activated carbon, thereby forming widened spaces between the particles of the porous material as compared to a conventional gas adsorber with no binder. The size of the particles is quite large at about 0.5 mm in diameter. The binder is provided in the form of a powdery resin material or a fibrous resin material. The plurality of particles and binder can be molded into any shape.
U.S. Pat. Publ. No. 2013/0170687 A1, the entire disclosure of which is herein incorporated by reference in its entirety, discloses a gas adsorbing material comprised of a plurality of zeolite particles adhered together by a binder to form grains of a zeolite material. The spacing between particles within the grains can be established by the binder and processing of the material. The zeolite particles are much smaller than the activated carbon particles, having a mean diameter below 10 micrometer. The average size of the grains of zeolite material is in the range between 0.2 millimeter and 0.9 millimeter. The resulting grains of zeolite material are large enough to allow for better physical handling over the use of the material in loose particle form and can be molded into convenient shapes for handling. An exemplar of such a gas adsorbing material is utilized in the N'Bass™ Virtual Back Volume Technology of Knowles Corporation. Several different miniaturized loudspeaker models incorporating the N'Bass™ technology are commercially available from Knowles.
Spherically shaped grains of zeolite material provide particular advantages in handling, packaging and space utilization. For example, spherical shaped grains of zeolite material have been added to the containers disclosed in U.S. Pat. Publ. No. 2013/0341118, resulting in more adsorbent material, and more surface area, being provided in a back volume than with other grain shapes. Spherically shaped grains of a zeolite material have also been directly filled into the back volume space of a loudspeaker device. The spherical shape particularly allows the grains to be “poured” into an opening in the back volume, which is then sealed after filling. While this method has obvious advantages, there is still a need to contain the spherical grains inside the back volume by use of a mesh or vent wall that is sound transparent. Additionally, the manufacturing processes required for this particular method, including placing the grains of zeolite material into the back volume, can be intricate and expensive. The alternative of using a container has the same disadvantages as disclosed above.
Gas adsorbing material has typically not been used in microphones, balanced armature receivers or other similar miniaturized acoustic transducer applications because the prior art methods have been inadequate or cost prohibitive given the much smaller available back volume spaces in those devices. Whereas U.S. Pat. Publ. No. 2013/0170687 A1 discloses a commercially available micro-loudspeaker having a back volume that measures 1 cm3, the entire volume of most balanced armature receivers used in in-ear earphones and hearing aids is less than one quarter of that size. And the available space into which gas adsorbing material could be added is a fraction of that small total space.
There is a desire therefore to provide the maximum possible adsorbency of a gas adsorbing material within the available space for a loudspeaker back volume within a mobile device. There is a further desire to use gas adsorbing materials to enhance the performance of acoustic transducers other than loudspeakers, such as microphones and balanced armature receivers which typically have even less space available to act as a back volume.